Unigue entry on the jam! It's a simple concept for all types of player as it test their observantness and reactions! Your idea fits the theme perfectly.
You added a tutorial screen on a short time which is a bonus!
More technical than most games in code (with encoders, decoders). I think a shorter message (or random messages with same letters / difficulty) will really help the replayability of the game and still test your players. I suggest a minute of gameplay. 